This invention relates generally to a catalyst system and method for the reduction of nitrogen oxide emissions and more particularly to a catalyst system that comprises a multi-component catalyst, a reductant and a compound comprising sulfur.
Methods have long been sought to reduce the deleterious effects of air pollution caused by byproducts resulting from the imperfect high-temperature combustion of organic materials. When combustion occurs in the presence of excess air and at high temperatures, harmful byproducts, such as nitrogen oxides, commonly known as NOx are created. NOx and subsequent derivatives have been suggested to play a major role in the formation of ground-level ozone that is associated with asthma and other respiratory ailments. NOx also contributes to soot formation, which is linked to a number of serious health effects, as well as to acid rain and the deterioration of coastal estuaries. As a result, NOx emissions are subject to many regulatory provisions limiting the amount of NOx that may be present in effluent gas vented into the surrounding environment.
One known method for dealing with NOx involves the use of selective catalytic reduction (SCR) to reduce NOx to nitrogen gas (N2) using ammonia (NH3) as a reductant. However, as ammonia's own hazardous consequences are well known, the use of NH3 in an SCR system presents additional environmental and other problems that must also be addressed. As regulatory agencies continue to drive limits on NOx emission lower, other regulations are also driving down the permissible levels of NH3 that may be emitted into the atmosphere. Because of regulatory limits on ammonia slip, the use of hydrocarbons and their oxygen derivatives for NOx reduction in a SCR process is very attractive. Numerous catalysts have been suggested for this purpose including zeolites, perovskites, and metals on metal oxide catalyst support. However, existing catalyst systems have either low activity or narrow region of working temperatures or low stability to water, which are detrimental to practical use. Furthermore catalysts active in NOx reduction are very sensitive to sulfur and lose their activity when sulfur is present in the system. For example, U.S. Pat. No. 6,703,343 teaches catalyst systems for use in NOx reduction. However, these catalyst systems require a specially synthesized metal oxide catalyst support with very low level of impurities. In addition, these catalyst systems are specifically sensitive to sulfur poisoning. Therefore there is a need for an effective catalyst system to reduce NOx emissions, which system is stable, operates at a wide range of temperatures, and operates effectively in the presence of sulfur.